Injection molding involves injecting molten thermoplastic resin into a mold apparatus. Molds for injection molding of thermoplastic resin are usually made from metal materials such as iron, steel, stainless steel, aluminum alloy or brass. Such materials are advantageous in that they have high thermal conductivity and thus allow the melt of thermoplastic resin to cool rapidly and shorten the molding cycle time. However, because of the rapid cooling, the injected resin freezes instantaneously at the mold surface, resulting in a thin solid layer. Quick quenching of the melt at the mold surface creates several problems. The freezing of these materials at the mold surfaces creates rough surface. Processing difficulties arise especially when producing thin parts requiring a high quality optical surface. The quick solidification of the melt combined with, for example, variable radial flowability of the materials makes it difficult to achieve the kind of uniform melt flow required for an optical disc. This is important when considering the quality of pits replication required for optical discs. Non-uniform flow can result in areas with high bit errors. The use of multiple gates is not generally thought to be a practical expedient means to remedy non-uniform melt flow in an optical medium, because weld lines are produced which can cause optical flaws.
For a process to be economical, a very careful balance must be maintained between low cycle times and the process parameters to meet the exacting quality standards. Under conventional methods these two production phenomena are usually working in opposition to each other.
Various types of molds have long been in use for preparing shaped articles from thermoplastic resins. Molds for these purposes are typically manufactured from metal or a similar material having high thermal conductivity. For most purposes, high thermal conductivity is desirable since it permits the resin in the mold to cool rapidly, shortening the molding cycle time. At times, however, cooling is so rapid that the resin freezes instantaneously at the mold surface upon introduction into the mold, forming a thin solid layer which, especially if it contains a filler, can create rough surfaces, voids, porosity and high levels of residual stress and orientation. In an optical disc, such imperfections impede the optical properties and decrease or eliminate performance.
There have recently been disclosed multilayer molds in which a metal core has an insulating layer bonded thereto for the purpose of slowing the initial cooling of the resin during the molding operation. The insulating layer is fabricated of material having low thermal diffusivity and conductivity, thus slowing the cooling of the molded resin, and also having good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. In order to improve the durability of the mold and to improve part surface quality, one or more skin layers of hard material, typically metal, is bonded to the insulating layer. The skin layer may be deposited by such operations as electroless deposition, electrolytic deposition and combinations thereof. Due to the insulation, the skin layer retains heat longer during the molding operation, thereby avoiding the surface irregularities created by rapid surface cooling.
Through thickness birefringence, (i.e., retardation and the influence of molding and specific process conditions on residual retardation or optical path difference) is also a crucial factor to be considered in connection with optical disc manufacturing.
The interrelationship of process conditions, birefringence, and pit replication is highly complex when manufacturing digital audio discs. The retardation profiles are a reliable measure of the effect which process conditions have on final optical properties. Circumferential variations reflect non-uniform heat transfer in the mold. Also, because the polycarbonate must cool against the nickel stamper with precise molding of the pits, heat transfer, here too, is important. Thus, improvement is required to render the heat transfer more uniform.